1. Field of the Invention
This invention relates to a method and apparatus for electrically stimulating the vagal nerves for the prevention of life threatening arrhythmias, either automatically or upon external command by the patient or physician.
2. Description of the Prior Art
vagal stimulation for the treatment of supraventricular arrhythmias, angina pectoris, and heart failure with an automatic, permanently implantable, medical device has been reported in the literature at least as far back as the early 1960's. The paper "Vagal Tuning" by Bilgutay et al. in the Journal of Thoracic and Cardiovascular Surgery, Vol. 56, No. 1, July 1968, pp 71-82 described the concepts of vagal stimulation in the treatment of supraventricular arrhythmias, angina pectoris, and heart failure employing an implantable vagal stimulator. Vagal stimulation was effected through the application of electrical stimulation to the vagus nerve by silastic coated, bipolar electrodes (of the type disclosed in Medtronic U.S. Pat. No. 3,421,511) surgically placed around the intact nerve or nerves. Bilgutay et al. designed and employed three different models of vagal stimulators, the first one having a magnetic switch by which it could be turned on or off from outside the body to study the effects of long term stimulation, the second type also implantable but powered from outside by induction using rf frequency, and the third, external type triggered by the R-wave of the subject's electrocardiogram to provide stimulation only upon an achievement of a certain heart rate. Bilgutay et al. found that when a pulsatile current with a frequency of ten pulses per second and 0.2 milliseconds pulse duration was applied to the vagus nerve, long term effective stimulation could be achieved, and also found that by increasing only the voltage amplitude and current, more predictable changes on the electrocardiogram could be obtained. The heart rate could be decreased down to half the resting rate while still preserving sinus rhythm up to 9 volts applied voltage. Atrial-Ventricular (AV) conduction dissociation and third degree heart block resulted at amplitudes exceeding 9 volts, and complete asystole with vagal escape resulted when the applied voltage exceeded 20 volts. Low amplitude vagal stimulation was successfully employed to control induced tachycardias and ectopic beats, and the authors noted other advantages of vagal stimulation in alleviating adverse effects of available drug therapies.
Other investigators reported treatment of angina pectoris and paroxysmal atrial ventricular junctional or supraventricular tachycardias through application of carotid sinus nerve stimulation employing the Medtronic Angistat carotid sinus nerve stimulator (csns) then available from Medtronic, Inc. in papers such as "Carotid Sinus Nerve Stimulation in the Treatment of Angina Pectoris and Supraventricular Tachycardia", California Medicine, 112:41-50, March, 1970, and in papers referenced therein. These papers describe the system for effecting carotid sinus nerve stimulation by a radio frequency responsive stimulator activated by the patient at will in response to the patient's awareness of the onset of angina and/or supraventricular tachycardia.
While these investigators focused upon the control of angina pectoris by regulation of the patient's blood pressure through stimulation of the carotid sinus nerve, they observed that the effect of the application of carotid sinus nerve stimulation provided a safe means for initiating reflex vagal activity which in turn effected a slowing in a patient's supraventricular tachycardia.
Thus, it is well known that the application of stimulation to the right or left vagus nerve, either directly or indirectly, has the effect of slowing tachycardias with the attendant possibility that a bradyarrhythmia may be induced in the process.
However, the difficulty in determining the proper amplitudes, frequencies and durations of the electrical stimulation indicated by Bilgutay et all coupled with continuing difficulty in the chronic vagal stimulation and other nerves has effectively led to the near abandonment of this therapy in recent years.